Big, blue bastards from Half-Life. They shoot flames from their arms and can stomp on the ground, sending some sort of red energy at you that can do considerable damage. Some have said that the gargantua cannot be killed with weapons. Do not believe this, because they can be killed, by using explosives or energy based weapons only. It's better to find another way to kill them, as they take quite some punishment before going down. They are rarely found in the game. They are not synthetic or machines of any kind. Sven Co-op has a smaller version of this creature, the Baby Gargantua.
As Gordon ran into the power room, the gargantua began to chase him. He could've used his grenades, but he didn't want to waste all 10 of them. Instead, he led him to the power room and switched on the power. he watched in satisfaction as the gargantua was fried by electricity.